The proposed research includes a study in laboratory mice and rats of the mechanisms and significance in the gastrointestinal ecosystem of the associations with mucosal epithelia of certain indigenous microorganisms. In addition, it includes efforts to determine the mechanisms by which various indigenous microbes influence certain biological properties of the animals. Microbiological culture techniques including methods for culturing strictly anaerobic bacteria, frozen-section histology, immunofluorescence techniques, and electron microscopy are used to characterize the epithelial associations. They are examined in ex-germfree mice associated with the layer forming microbes. The microbial culture and microscopic techniques are also used along with biochemical methods (thin-layer and gas-liquid chromatography, enzymatic assays, acrylamide gel electrophoresis) in experiments on the mechanisms by which indigenous microbes influence host cecal size, bile acid composition, and intestinal alkaline phosphatase activity. These properties are examined in adult and baby specific pathogen-free and ex-germfree mice associated with particular indigenous microbes. The microbes tested are selected because they have anaerobic metabolism, colonize the animals early in life, maintain high population levels throughout life, and associate with specific epithelia.